The subject matter disclosed herein relates generally to magnetic resonance imaging systems, and more particularly to systems and methods for designing radio-frequency (RF) pulses for magnetic resonance imaging (MRI) systems.
MRI systems include a magnet, such as a superconducting magnet that generates a temporally constant (i.e., uniform and static) primary or main magnetic field. MRI data acquisition is accomplished by exciting magnetic moments within the primary magnetic field using transmit radio-frequency coils. For example, in order to image a region of interest, the magnetic gradient coils are energized to impose a magnetic field gradient to the primary magnetic field. Transmit radio-frequency (RF) coils are then pulsed to create RF magnetic field pulses in a bore of an MRI scanner to selectively excite a volume corresponding to the region of interest in order to acquire MR images of the region of interest using receive RF coils. During the transmission of the RF magnetic field pulses, the receive RF coils are decoupled or disabled and during reception the transmit RF coils are decoupled or disabled. In one example, these are the same coils that are switched from a transmit to a receive mode. The resultant magnetic resonance (MR) image that is generated shows the structure and/or function of the region of interest.
MR images, thus, may be created by applying currents to the gradient and RF coils according to defined pulse sequences. Selection of a pulse sequence, for example, by an operator, determines the relative appearance of different tissue types in the resultant images, emphasizing or suppressing tissue types as desired.
The RF pulse sequences are typically designed to achieve the desired characteristic for imaging. The Shinnar-Le Roux (SLR) RF pulse design algorithm is currently the most widely-used method for designing large-tip-angle one-dimensional RF pulses on constant gradient waveforms. Although the SLR RF pulse design algorithm simplifies the design of the input (RF pulse) into a linear problem, namely a design of two linear finite impulse response (FIR) filters, such that designed filters have a one-to-one correspondence to an RF pulse, and are easily inverted to obtain that pulse, the SLR algorithm is limited in design capabilities. In particular, the SLR algorithm is presently limited in that the algorithm is only capable of designing one-dimensional pulses for constant gradient trajectories. The constant gradient limitation arises because non-constant gradients require the design of filters with up to a computationally-prohibitive 2Nt−1 nonuniformly-spaced taps, the coefficients of which must be designed while using only the degrees of freedom afforded by the Nt samples in the RF pulse. Thus, the SLR algorithm cannot be used for designing multidimensional or one-dimensional pulses on non-constant gradient waveforms.